1. Field of the Invention
The invention relates to field emitters, and more particularly to exposed wide band gap field emission areas and a method of making same.
2. Description of Related Art
Field emitters are widely used as sources of electrons in lamps and scanning electron microscopes since emission is affected by the adsorbed materials. Field emitters have also been found useful in flat panel displays and vacuum microelectronics applications. Cold cathode and field emission based flat panel displays have several advantages over other types of flat panel displays, including low power dissipation, high intensity and low projected cost. Thus, an improved field emitter and any process which reduces the complexity of fabricating field emitters is clearly useful.
The present invention can be better appreciated with an understanding of the related physics. General electron emission can be analogized to the ionization of a free atom. Prior to ionization, the energy of electrons in an atom is lower than electrons at rest in a vacuum. In order to ionize the atom, energy must be supplied to the electrons in the atom. That is, the atom fails to spontaneously emit electrons unless the electrons are provided with energy greater than or equal to the electrons at rest in the vacuum. Energy can be provided by numerous means, such as by heat or irradiation with light. When sufficient energy is imparted to the atom, ionization occurs and the atom releases one or more electrons.
Several types of electron emission are known. Thermionic emission involves an electrically charged particle emitted by an incandescent substance (as in a vacuum tube or incandescent light bulb). Photoemission releases electrons from a material by means of energy supplied by incidence of radiation, especially light. Secondary emission occurs by bombardment of a substance with charged particles such as electrons or ions. Electron injection involves the emission from one solid to another. Finally, field emission refers to the emission of electrons due to an electric field.
In field emission (or cold emission), electrons under the influence of a strong electric field are liberated out of a substance (usually a metal or semiconductor) into a dielectric (usually a vacuum). The electrons "tunnel" through a potential barrier instead of escaping "over" it as in thermionics or photoemission. Field emission is therefore a quantum-mechanics phenomena with no classical analog. A more detailed discussion of the physics of field emission can be found in U.S. Pat. No. 4,663,559 to Christensen; Cade and Lee, "Vacuum Microelectronics", GEC J. Res. Inc., Marconi Rev., 7(3), 129 (1990); and Cutler and Tsong, Field Emission and Related Topics (1978).
The shape of a field emitter affects its emission characteristics. Field emission is most easily obtained from sharply pointed needles or tips whose ends have been smoothed into a nearly hemispherical shape by heating. Tip radii as small as 100 nanometers have been reported. As an electric field is applied, the electric lines of force diverge radially from the tip and the emitted electron trajectories initially follow these lines of force. Field emitters with such sharp features similar to a "Spindt cathode" have been previously invented. An overview of vacuum electronics and Spindt type cathodes is found in the November and December, 1989 issues of IEEE Transactions of Electronic Devices. Fabrication of such fine tips, however, normally requires extensive fabrication facilities to finely tailor the emitter into a conical shape. Further, it is difficult to build large area field emitters since the cone size is limited by the lithographic equipment. It is also difficult to perform fine feature lithography on large area substrates as required by flat panel display type applications. Thus, there is a need for a method of making field emitters with fine conical or pyramid shaped features without the use of lithography.
The work function of the electron emitting surface or tip of a field emitter also effects emission characteristics. The work function is defined as the difference in energies of the Fermi level and vacuum level. A smaller work function requires lower voltage to emit electrons from a surface. In a metal, the Fermi level is the same as the conduction band. In wide band gap materials, however, the Fermi level lies between the conduction band and the valence band. In such a case, the work function of the material changes as the Fermi level changes due to doping or defects. Further, the energy difference between the conduction band and vacuum level is a fundamental material property referred to as electron affinity. Thus, the work function and electron affinity are the same in a metal, but different in a wide band gap material. Recently, several wide band gap semiconductors (insulators at room temperature) such as diamond and aluminum-nitride have been shown to have negative electron affinity as well. See, for example, Yoder, "Applications of Diamond and Related Materials", 5th Annual Diamond Technology Workshop, Troy, Mich., May 18-20, 1994; Davis, "Growth and Characterization of III-V Nitride Thin Films via Plasma-and Ion-assisted Gas-source Molecular Beam Epitaxy", 5th Annual Diamond Technology Workshop, Troy, Mich., May 18-20, 1994; Rubin et al., "P-Type Gallium Nitride by Reactive Ion-Beam Molecular Beam Epitaxy with Ion Implantation, Diffusion or Coevaporation of Mg", pre-print by Lawrence Berkeley Laboratory, University of California, Berkeley, Calif., March 1994, pp. 1-7; and Newman et al., "Thermodynamic and Kinetic Processes Involved in the Growth of Epitaxial GaN Thin Films", Applied Physics Letters, 62 (11), 15 March 1993, pp. 1242-1244.
There are other materials which exhibit low or negative electron affinity, but almost all of these materials are alkali metal based. Alkali metals are quite sensitive to atmospheric conditions and tend to decompose when exposed to air or moisture. Additionally, alkali metals have low melting points, typically below 1000.degree. C., which may be unsuitable in certain applications.
For a full understanding of the prior art related to the present invention, certain attributes of diamond must also be discussed. Recently, it has been experimentally confirmed that the (111) surface of diamond crystal has an electron affinity of -0.7+/-0.5 electron-volts, showing it to possess negative electron affinity. A common conception about diamonds is that they are very expensive to fabricate. This is not always the case, however. Newly invented plasma chemical vapor deposition processes appear to be promising ways to bring down the cost of producing high quality diamond thin films. For instance, high fidelity audio speakers with diamond thin films as vibrating cones are already commercially available. It should also be noted that diamond thin films cost far less than the high quality diamonds used in jewelry.
Diamond cold cathodes have been reported by Geis et al. in "Diamond Cold Cathode", IEEE Electron Device Letters, Vol. 12, No. 8, August 1991, pp. 456-459; and in "Diamond Cold Cathodes", Applications of Diamond Films and Related Materials, Tzeng et al. (Editors), Elsevier Science Publishers B.V., 1991, pp. 309-310. The diamond cold cathodes are formed by fabricating mesa-etched diodes using carbon ion implantation into p-type diamond substrates. Geis et al. indicate that the diamond can be doped either n- or p-type. In fact, several methods show promise for fabricating n-type diamond, such as bombarding the film with sodium, nitrogen or lithium during growth. However, in current practice it is extremely difficult to fabricate n-type diamond and efforts for n-type doping usually result in p-type diamond. Furthermore, p-type doping fails to take full advantage of the negative electron affinity effect, and pure or undoped diamond is insulating and normally charges up to prevent emission.
There exists a need for improved methods of making field emission areas as well as improved field emitter structures using diamond and other wide band gap materials.